A measurement probe is an apparatus that detects signals on a device under test (DUT) and transmits those signals to a measurement instrument. For example, a current probe detects an amount of current flowing through a DUT and transmits a signal indicating the detected current to a measurement instrument such as an oscilloscope.
In certain applications, it may be desirable for a measurement probe to detect signals over a large range of values, i.e., over a high dynamic range. For instance, when characterizing a mobile phone, it may be desirable to detect its current during a low current state, such as a steep state, and during a high current state, such as a signal transmission state. Moreover, it may also be desirable to observe signals at varying levels of scope or resolution, e.g., at a zoomed-in level and a zoomed-out level.
Unfortunately, conventional measurement probes tend to suffer from various shortcomings when it comes to performing measurements over a high dynamic range. As examples, consider conventional current probes used with oscilloscopes. These current probes generally fall into one of two topologies. In a first topology, a low value current sense resistor is placed in series with conductor carrying a current to be measured and then a high impedance differential voltage amplifier is used to measure a voltage across the current sense resistor and deliver a proportional voltage signal to an oscilloscope. In a second topology, a combination Hall effect sensor and transformer are used to sense a magnetic field from a conductor carrying a current and the sensed voltage is amplified to deliver a proportional voltage signal to an oscilloscope. The Hall effect sensor senses from direct current (DC) to some fairly low bandwidth (BW) point and the transformer senses from the low BW point to a full BW of the probe. These two signals are combined to achieve a DC to full bandwidth design.
These conventional current probes generally have limited dynamic range due to noise. In particular, they typically have a noise floor that is constrained by a maximum current they can sense. Accordingly, because their signal to noise ratio (SNR) is limited, if a maximum input range is picked to cover the largest signals, then the equivalent input noise is too large to allow measurement of very small signals.
In consideration of these and other shortcomings of conventional measurement probes, there is a general need for improved approaches for detecting signals over a high dynamic range.